3S MOEPHOLOGY AND EVOLUTION
The head is rounded, and devoid of a prominent
Hands and feet (figs. 7 and 8j.—The chief points of note regarding the hands and feet of the Hapalidse are the shortness and non-opposability of the pollex, the variation in the amount of syndactyly, and the presence of strong claws on all digits except the halluxes, which have true nails. And the first and third of these characters distinguish the Hapalidae from the Cebidae. The claws enable the animals to secure a firm hold on the branches, for the digits have a feeble grasping power. In the Cebidae the pollex is absent in Ateles, but it is present in other genera. When present it is short, and its degree of opposability varies, but is never very great. With few exceptions syndactyly does not exist. It is frequently difficult to determine whether the digits have claws or nails. The hallux is well developed.
The pads are best marked in the primitive, nocturnal DouroucouliSj and Pocock (172) suggests that the strong tactile sense compensates for imperfect nocturnal vision.
The proportions of the hands and feet vary considerably, and those of Ateles approach those of Simla; both animals use their hands in a hook-like manner.
The Tail—All Platyrrhini have tails, but it is prehensile in only half of the total genera. It is prehensile in Ateles, Bmchyteles,Alouatta, Gebus and Lagothrix; but it is not prehensile in the other Gebidse nor in the Hapalidse, When the tail is prehensile it exhibits variations in the degree of tactile sensibility.
Hair.—All Platyrrhini have a well-marked hairy covering and the fur varies in texture, feel and colour.